Severe air pollution might have killed millions worldwide

Air pollution has become a major health issue worldwide, and now, new research suggests it's linked to 5.5 million premature deaths.

According to data from the Global Burden of Disease project, air pollution, both in the home and outside, ranked as the fourth highest contributor to death worldwide.

China and India topped the list of countries with the majority of deaths, with 1.6 million and 1.4 million respectively.

Click through images of smog in China and India:

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Smog in China and India, air pollution

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Severe air pollution might have killed millions worldwide

Commuters drive on a road in heavy pollution in Beijing on December 1, 2015. Beijing ordered hundreds of factories to shut and allowed children to skip school as choking smog reached over 25 times safe levels on December 1, casting a cloud over China's participation in Paris climate talks. AFP PHOTO / WANG ZHAO / AFP / WANG ZHAO (Photo credit should read WANG ZHAO/AFP/Getty Images)

Commuters wearing protection masks wait for buses at a bus stand on a polluted day in Beijing, Monday, Dec. 7, 2015. Smog shrouded the capital city Monday after authorities in Beijing issued an orange alert on Saturday. (AP Photo/Andy Wong)

BEIJING, CHINA - DECEMBER 01: A woman wearing a mask practices roller blading at Olympic Park during dheavy smog on December 1, 2015 in Beijing, China. The representatives of the governments of more than 190 countries are meeting in Paris this week, including Chinese President Xi Jinping will attend the Paris meeting. (Photo by Feng Li/Getty Images)

A woman uses her scarf to cover her mouth as she crosses a street in Beijing on December 1, 2015. Beijing ordered hundreds of factories to shut and allowed children to skip school as choking smog reached over 25 times safe levels on December 1, casting a cloud over China's participation in Paris climate talks. AFP PHOTO / WANG ZHAO / AFP / WANG ZHAO (Photo credit should read WANG ZHAO/AFP/Getty Images)

BEIJING, CHINA - DECEMBER 01: A man rides riding a tricycle with plastic bottles to be recycled on a day of heavy pollution on December 1, 2015 in Beijing, China. China's capital and many cities in the northern part of the country recorded the worst smog of the year with air quality devices in some areas unable to read such high levels of pollutants. Levels of PM 2.5, considered the most hazardous, crossed 600 units in Beijing, nearly 25 times the acceptable standard set by the World Health Organization. The governments of more than 190 countries are meeting in Paris this week to set targets on reducing carbon emissions in an attempt to forge a new global agreement on climate change. (Photo by Lintao Zhang/Getty Images)

Commuters drive on a road in heavy pollution in Beijing on December 1, 2015. Beijing ordered hundreds of factories to shut and allowed children to skip school as choking smog reached over 25 times safe levels on December 1, casting a cloud over China's participation in Paris climate talks. AFP PHOTO / WANG ZHAO / AFP / WANG ZHAO (Photo credit should read WANG ZHAO/AFP/Getty Images)

BEIJING, CHINA - DECEMBER 01: A subway passing bridge on a day of heavy pollution on December 1, 2015 in Beijing, China. China's capital and many cities in the northern part of the country recorded the worst smog of the year with air quality devices in some areas unable to read such high levels of pollutants. Levels of PM 2.5, considered the most hazardous, crossed 600 units in Beijing, nearly 25 times the acceptable standard set by the World Health Organization. The governments of more than 190 countries are meeting in Paris this week to set targets on reducing carbon emissions in an attempt to forge a new global agreement on climate change. (Photo by Lintao Zhang/Getty Images)

People arrive at the airport on a polluted day in Beijing on December 1, 2015. China has ordered thousands of factories to shut as it grapples with swathes of choking smog that were nearly 24 times safe levels on December 1, casting a shadow over the country's participation in Paris climate talks. A thick grey haze shrouded Beijing, with the concentration of PM 2.5, harmful microscopic particles that penetrate deep into the lungs, climbing as high as 598 micrograms per cubic metre AFP PHOTO / FRED DUFOUR / AFP / FRED DUFOUR (Photo credit should read FRED DUFOUR/AFP/Getty Images)

Visitors wear masks as they walk in Tiananmen Square during heavy pollution in Beijing on December 1, 2015. Beijing ordered hundreds of factories to shut and allowed children to skip school as choking smog reached over 25 times safe levels on December 1, casting a cloud over China's participation in Paris climate talks. AFP PHOTO / WANG ZHAO / AFP / WANG ZHAO (Photo credit should read WANG ZHAO/AFP/Getty Images)

People walk through a overpass in Beijing on December 1, 2015. Beijing ordered hundreds of factories to shut and allowed children to skip school as choking smog reached over 25 times safe levels on December 1, casting a cloud over China's participation in Paris climate talks. AFP PHOTO / WANG ZHAO / AFP / WANG ZHAO (Photo credit should read WANG ZHAO/AFP/Getty Images)

Pedestrians walk past a billboard scene of green trees and grass on a heavily polluted day in Beijing on December 1, 2015. China has ordered thousands of factories to shut as it grapples with swathes of choking smog that were nearly 24 times safe levels on December 1, casting a shadow over the country's participation in Paris climate talks. A thick grey haze shrouded Beijing, with the concentration of PM 2.5, harmful microscopic particles that penetrate deep into the lungs, climbing as high as 598 micrograms per cubic metre. AFP PHOTO / GREG BAKER / AFP / GREG BAKER (Photo credit should read GREG BAKER/AFP/Getty Images)

BEIJING, CHINA - DECEMBER 01: Chinese Tai Chi practitioners exercise at a local park on a high pollution day on December 1, 2015 in Beijing, China. China's capital and many cities in the northern part of the country recorded the worst smog of the year with air quality devices in some areas unable to read such high levels of pollutants. Levels of PM 2.5, considered the most hazardous, crossed 600 units in Beijing, nearly 25 times the acceptable standard set by the World Health Organization. The governments of more than 190 countries are meeting in Paris this week to set targets on reducing carbon emissions in an attempt to forge a new global agreement on climate change. (Photo by Kevin Frayer/Getty Images)

BEIJING, CHINA - NOVEMBER 30: A Chinese man wears a protective face mask as he passes by the CCTV building on a day of heavy pollution on November 30, 2015 in Beijing, China. China's capital and many cities in the northern part of the country recorded the worst smog of the year with air quality devices in some areas unable to read such high levels of pollutants. Levels of PM 2.5, considered the most hazardous, crossed 600 units in Beijing, nearly 25 times the acceptable standard set by the World Health Organization. The governments of more than 190 countries are meeting in Paris this week to set targets on reducing carbon emissions in an attempt to forge a new global agreement on climate change.(Photo by Kevin Frayer/Getty Images)

BEIJING, CHINA - DECEMBER 01: Tourists visit the Olympic Park during dheavy smog on December 1, 2015 in Beijing, China. The representatives of the governments of more than 190 countries are meeting in Paris this week, including Chinese President Xi Jinping will attend the Paris meeting. (Photo by Li Feng/Getty Images)

This combination of photos taken Sunday, Nov. 1, 2015, top, and Tuesday, Dec. 1, 2015, bottom, show pedestrians walking through a shopping and office complex in Beijing amid widely differing levels of air pollution. Schools in Beijing were ordered to keep students indoors Tuesday, Dec. 1, 2015 after record-breaking air pollution in the Chinese capital soared to up to 35 times the safe levels. (AP Photo/Mark Schiefelbein)

This combination of photos taken Sunday, Nov. 1, 2015, top, and Tuesday, Dec. 1, 2015, bottom, show pedestrians walking past an elevated highway in Beijing amid widely differing levels of air pollution. Schools in Beijing were ordered to keep students indoors Tuesday, Dec. 1, 2015 after record-breaking air pollution in the Chinese capital soared to up to 35 times safe levels. (AP Photo/Mark Schiefelbein)

This combination of photos taken Sunday, Nov. 1, 2015, top, and Tuesday, Dec. 1, 2015, bottom, show pedestrians walking through a shopping and office complex in Beijing amid widely differing levels of air pollution. Schools in Beijing were ordered to keep students indoors Tuesday, Dec. 1, 2015 after record-breaking air pollution in the Chinese capital soared to up to 35 times safe levels. (AP Photo/Mark Schiefelbein)

This combination image of two photographs taken on December 3, 2015 (top) and two days earlier on December 1 (bottom) shows a skyscraper under clear skies and in heavy pollution, as seen in the central business district in Beijing. The skies cleared in Beijing on December 2 and 3, after being swathed in choking smog that was nearly 24 times safe levels earlier in the week. AFP PHOTO / GREG BAKER / AFP / GREG BAKER (Photo credit should read GREG BAKER/AFP/Getty Images)

This photo combo shows a general view of people visiting Tiananmen Square during a heavily polluted day on December 1, 2015 (LOWER) and two days later when the smog cleared on December 3 (TOP) in Beijing. The skies cleared in Beijing on December 2 and 3, after being swathed in choking smog that was nearly 24 times safe levels earlier in the week. AFP PHOTO / WANG ZHAO
/ AFP / WANG ZHAO (Photo credit should read WANG ZHAO/AFP/Getty Images)

A woman wearing a mask exits an underpass in Beijing on November 30, 2015. Beijing choked under the worst smog of the year on November 30, with dangerous particulates nearly 20 times healthy levels, as China's president joined other leaders in Paris for key climate change talks. AFP PHOTO / WANG ZHAO / AFP / WANG ZHAO (Photo credit should read WANG ZHAO/AFP/Getty Images)

BEIJING, CHINA - NOVEMBER 30: A couple wear protective masks as they have their picture taken outside the Forbidden City on a day of heavy pollution on November 30, 2015 in Beijing, China. China's capital and many cities in the northern part of the country recorded the worst smog of the year with air quality devices in some areas unable to read such high levels of pollutants. Levels of PM 2.5, considered the most hazardous, crossed 600 units in Beijing, nearly 25 times the acceptable standard set by the World Health Organization. The governments of more than 190 countries are meeting in Paris this week to set targets on reducing carbon emissions in an attempt to forge a new global agreement on climate change. (Photo by Kevin Frayer/Getty Images)

SHANGHAI, CHINA - NOVEMBER 30: (CHINA OUT) Buildings at the Lujiazui Financial District are shrouded in smog on November 30, 2015 in Shanghai, China. Shanghai's real-time Air pollution index (API) read high 189 on Monday morning. (Photo by ChinaFotoPress/ChinaFotoPress via Getty Images)

Pedestrians walk over a footbridge shrouded in haze as vehicles drive on a highway in Beijing, China, on Monday, Nov. 30, 2015. Air pollution reached 'hazardous' levels in Beijing on Sunday, prompting the city to upgrade to the second-highest alert for the first time in 13 months on the same day that the Chinese government said it has met pollution-reduction targets for the year. Photographer: Qilai Shen/Bloomberg via Getty Images

BEIJING, CHINA - NOVEMBER 30: A Chinese woman wears a protective mask as she walks outside the Forbidden City on a day of heavy pollution on November 30, 2015 in Beijing, China. China's capital and many cities in the northern part of the country recorded the worst smog of the year with air quality devices in some areas unable to read such high levels of pollutants. Levels of PM 2.5, considered the most hazardous, crossed 600 units in Beijing, nearly 25 times the acceptable standard set by the World Health Organization. The governments of more than 190 countries are meeting in Paris this week to set targets on reducing carbon emissions in an attempt to forge a new global agreement on climate change. (Photo by Kevin Frayer/Getty Images)

A woman wearing a mask walks past in Beijing on November 30, 2015. Beijing choked under the worst smog of the year on November 30, with dangerous particulates nearly 20 times healthy levels, as China's president joined other leaders in Paris for key climate change talks. AFP PHOTO / WANG ZHAO / AFP / WANG ZHAO (Photo credit should read WANG ZHAO/AFP/Getty Images)

A man wears a mask on a polluted day in Beijing on November 30, 2015. Beijing choked under the worst smog of the year on November 30, with dangerous particulates nearly 20 times healthy levels, as China's president joined other leaders in Paris for key climate change talks. AFP PHOTO / FRED DUFOUR / AFP / FRED DUFOUR (Photo credit should read FRED DUFOUR/AFP/Getty Images)

To go with Climate-warming-UN-COP21-India,FOCUS by Trudy Harris
In this November 15, 2015 photo, Indian joggers exercise on a smoggy morning near the India Gate monument in New Delhi. India's capital, with 18 million residents, has the world's most polluted air with six times the amount of small particulate matter (pm2.5) than what is considered safe, according to the World Health Organization (WHO). The air's hazardous amount of pm2.5 can reach deep into the lungs and enter the blood, causing serious long term health effect, with the WHO warning India has the world's highest death rate from chronic respiratory diseases. India, home to 13 of the world's top 20 polluted cities, is also the third largest emitter of greenhouse gases behind the United States and China. In Delhi, the air pollution is due to vehicle traffic including cargo trucks running on low-grade diesel, individual fires that residents burn in winter, crop being burnt by farmers in neighboring states, and construction site dust. Burning coal in power plants is also major contributor that is expected to increase hugely in the coming decades to match electricity needs of the ever-growing city and its booming satellite towns. (Photo credit should read ROBERTO SCHMIDT/AFP/Getty Images)

An Indian schoolgirl covers her nose and mouth as she walks with others along a busy road on a smoggy day in New Delhi on November 30, 2015. Some 150 leaders including US President Barack Obama, China's Xi Jinping, India's Narendra Modi and Russian President Vladimir Putin will attend the start of the Paris conference on climate change, which starts on November 30, tasked with reaching the first truly universal climate pact. AFP PHOTO / Money SHARMA / AFP / MONEY SHARMA (Photo credit should read MONEY SHARMA/AFP/Getty Images)

Heavy traffic is seen during a smoggy day in New Delhi on November 30, 2015. Some 150 leaders including US President Barack Obama, China's Xi Jinping, India's Narendra Modi and Russian President Vladimir Putin will attend the start of the Paris conference on climate change, which starts on November 30, tasked with reaching the first truly universal climate pact. AFP PHOTO / Money SHARMA / AFP / MONEY SHARMA (Photo credit should read MONEY SHARMA/AFP/Getty Images)

BEIJING, CHINA - NOVEMBER 30: A residence community is blanketed by smog on November 30, 2015 in Beijing, China. (Photo by Xiao Lu Chu/Getty Images)

A man rides a bicycle along a road on a heavily polluted day in Beijing, Sunday, Nov. 29, 2015. Beijing on Sunday issued its highest smog alert of the year following air pollution in capital city reached hazardous levels as smog engulfed large parts of the country despite efforts to clean up the foul air. (AP Photo/Andy Wong)

A man walks on a pavement on a heavily polluted day in Beijing, Sunday, Nov. 29, 2015. Beijing on Sunday issued its highest smog alert of the year following air pollution in capital city reached hazardous levels as smog engulfed large parts of the country despite efforts to clean up the foul air. (AP Photo/Andy Wong)

A man wears a mask on a polluted day in Beijing on November 30, 2015. Beijing choked under the worst smog of the year on November 30, with dangerous particulates nearly 20 times healthy levels, as China's president joined other leaders in Paris for key climate change talks. AFP PHOTO / FRED DUFOUR / AFP / FRED DUFOUR (Photo credit should read FRED DUFOUR/AFP/Getty Images)

A Chinese paramilitary policeman stands guard on Tiananmen Square on a day with poor air quality in Beijing, Saturday, Nov. 28, 2015. Air pollution in Beijing reached hazardous levels on Saturday as smog engulfed large parts of the country despite efforts to clean up the foul air. (AP Photo/Mark Schiefelbein)

A bird flies on a smog filled morning in New Delhi, India, Wednesday, Nov.11, 2015. Delhi's air quality has been hitting new lows over the past week and authorities are urging people to refrain from lighting smoke emanating firecrackers during the upcoming Diwali festival. (AP Photo/Tsering Topgyal)

SHANGHAI, CHINA - NOVEMBER 30: (CHINA OUT) People take photos at The Bund in smog on November 30, 2015 in Shanghai, China. Shanghai's real-time Air pollution index (API) read high 189 on Monday morning. (Photo by ChinaFotoPress/ChinaFotoPress via Getty Images)

People wearing face masks walk across Tiananmen Square on a day with poor air quality in Beijing, Saturday, Nov. 28, 2015. Air pollution in Beijing reached hazardous levels on Saturday as smog engulfed large parts of the country despite efforts to clean up the foul air. (AP Photo/Mark Schiefelbein)

Heavy traffic is seen during a smoggy day in New Delhi on November 30, 2015. Some 150 leaders including US President Barack Obama, China's Xi Jinping, India's Narendra Modi and Russian President Vladimir Putin will attend the start of the Paris conference on climate change, which starts on November 30, tasked with reaching the first truly universal climate pact. AFP PHOTO / Money SHARMA / AFP / MONEY SHARMA (Photo credit should read MONEY SHARMA/AFP/Getty Images)

Vehicles drive along a road with a traffic sign reading "Visibility low, slowdown the speed" on a heavily polluted day in Beijing, Monday, Nov. 30, 2015. Beijing on Sunday, Nov. 29 issued its highest smog alert of the year following air pollution in capital city reached hazardous levels as smog engulfed large parts of the country despite efforts to clean up the foul air. (AP Photo/Andy Wong)

SHANGHAI, CHINA - NOVEMBER 30: (CHINA OUT) Buildings at the Lujiazui Financial District are shrouded in smog on November 30, 2015 in Shanghai, China. Shanghai's real-time Air pollution index (API) read high 189 on Monday morning. (Photo by ChinaFotoPress/ChinaFotoPress via Getty Images)

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"One of the reasons we're focused particularly on China and India is that the sources that contribute to air pollution are also major sources contributing to climate change," said a professor at the University of British Columbia.

Those findings come from 2013, the most recent year that data is available. But Beijing's smog levels have made headlines for years. As pollution levels rose higher at the end of 2015, the government warned people to stay inside.

And this past November in India, New Delhi issued a severe warning of its own for air pollution.

For both countries, estimates say pollution levels are expected to continue rising through 2030 despite commitments to curb the problem.